1. Field of the Invention
Embodiments of the invention described herein pertain to the field of floating liquefaction of natural gas. More particularly, but not by way of limitation, one or more embodiments of the invention enable a floating liquefied natural gas commissioning system and method.
2. Description of the Related Art
Natural gas is often transported by seagoing vessel from the location where it is produced to the location where it is consumed. Liquefaction of natural gas facilitates storage and transportation of the natural gas, since liquefied natural gas (“LNG”) takes up only about 1/600 of the volume that the same amount of natural gas does in its gaseous state. LNG is produced by cooling natural gas below its boiling point (−259° F. at atmospheric pressure). LNG may be stored in cryogenic containers slightly above atmospheric pressure. By raising the temperature of the LNG, it may be converted back to its gaseous form.
Natural gas produced in locations where it is abundant, may be liquefied and shipped overseas in this manner to locations where it is most needed. Typically, the natural gas is gathered through one or more pipelines to a liquefaction facility. Land-based liquefaction facilities and the associated gathering pipelines are costly, may occupy large areas of land, take several years to permit and construct and may not be practical or economical for offshore stranded natural gas reserves. Thus, demand has stimulated the development of floating natural gas liquefaction (“FLNG”).
FLNG facilities capable of producing, storing and offloading LNG, are typically autonomous floating structures including a vessel hull and liquefaction trains on deck. FLNG vessels are constructed in a shipyard, and then towed to a destination site where they may be integrated with the gas source. Prior to being placed in operation, an FLNG vessel must be properly commissioned. Commissioning involves the testing of all the equipment onboard the FLNG vessel, including the liquefaction facilities, in order to ensure proper operation prior to the vessel being put into service.
It has been proposed that commissioning take place at the destination site where a gas source is readily available. However, the destination site may be thousands of miles from the shipyard, and in the instance that any problems arise during commissioning, the FLNG vessel will be far from needed testing equipment and parts that are typically only available in the shipyard. Towing the FLNG vessel back to the shipyard would be time consuming and expensive, potentially setting a liquefaction project irreparably off-schedule or over-budget.
It has also been proposed that commissioning take place at an offshore location close to the shipyard. In this latter instance, the problem arises that there is no readily available gas source near the shipyard for use in testing the liquefaction trains. One suggestion has been to use LNG carriers to transport LNG to the commissioning site. However, the drawback to this solution is that the natural gas onboard conventional LNG carriers is already in liquefied form. Thus, the only gas available for use in testing the FLNG liquefaction trains is natural boil-off gas (“BOG”) located in the cargo tanks of the LNG carriers. The use of BOG for commissioning is not ideal because BOG is typically at a pressure of 2-3 bar, whereas during normal operation of the FLNG, the gas to be liquefied would be at about 45-100 bar. Thus, use of BOG for commissioning means accepting commissioning at lower than normal operating pressures, or boosting the pressure of the BOG prior to its use in testing—neither of which is a satisfactory solution due to added costs or risk of improper commissioning conditions.
Therefore, there is a need for a floating liquefied natural gas commissioning system and method.